Description

This frequency relay (81) includes three types of frequency elements, namely the overfrequency (81O), the under-frequency (81U), and the rate-of-change-of-frequency (ROCOF) (81R) elements. There are five levels of thresholds available with the 81O and 81U elements, whereas the 81R element features only one level.

Each level corresponds to a predefined frequency level and a tripping delay time. When the frequency or ROCOF monitored by the relay exceeds the settings, the relay operates. The model works for 50 Hz or 60 Hz system

Table of Contents

Model Overview

The frequency relay operates whenever the frequency or ROCOF measured from the input voltage signals using a zero-crossing detection logic exceeds the preset settings.

A time delay is used to enable the relay to ride through momentary frequency fluctuations and thus prevent nuisance operation. The five-level characteristic of 81O and 81U are presented in Figures as shown below:

81O element characteristic

Characteristics of 81O

81U element characteristic

Characteristics of 81U

Mask and Parameters

General Parameters

The following parameters are tunable:

Name	Description	Unit	Variable = {Possible Values}

Name	Description	Unit	Variable = {Possible Values}
Frequency	The nominal frequency of the measured voltage signals. Both 50 and 60 Hz are supported in the model.	Hz	Freq_param= {50, 60}
Freezing time	The freezing time of the 81 relay, when t < freezing time, the 81 relay is not activated.	s	tunf_param = { [0.10, 60.00] }

Settings - Overfrequency

The following parameters are tunable:

Name	Description	Unit	Variable = {Possible Values}

Name	Description	Unit	Variable = {Possible Values}
Enable Overfrequency	Overfrequency element enable/disable	-	Enable_81O_param= {0, 1}
Level 1	Over-frequency level 1.	Hz	OLevel1_param={ [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 2	Over-frequency level 2.	Hz	OLevel2_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 3	Over-frequency level 3.	Hz	OLevel3_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 4	Over-frequency level 4.	Hz	OLevel4_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 5	Over-frequency level 5.	Hz	OLevel5_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Time 1	Time delay associated with Level 1.	s	OTime1_param= { [0, 600] }
Time 2	Time delay associated with Level 2.	s	OTime2_param= { [0, 600] }
Time 3	Time delay associated with Level 3.	s	OTime3_param= { [0, 600] }
Time 4	Time delay associated with Level 4.	s	OTime4_param= { [0, 600] }
Time 5	Time delay associated with Level 5.	s	OTime5_param= { [0, 600] }

Settings - Underfrequency

The following settings are tunable:

Name	Description	Unit	Variable = {Possible Values}

Name	Description	Unit	Variable = {Possible Values}
Enable Underfrequency	Underfrequency element enable/disable	-	Enable_81U_param= {0, 1}
Level 1	Under-frequency level 1.	Hz	ULevel1_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 1	Under-frequency level 2.	Hz	ULevel2_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 1	Under-frequency level 3.	Hz	ULevel3_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 1	Under-frequency level 4.	Hz	ULevel4_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Level 1	Under-frequency level 5.	Hz	ULevel5_param= { [45, 55] } for 50 Hz Operation; { [55, 65] } for 60 Hz Operation.
Time 1	Time delay associated with Level 1.	s	UTime1_param= { [0, 600] }
Time 1	Time delay associated with Level 2.	s	UTime2_param= { [0, 600] }
Time 1	Time delay associated with Level 3.	s	UTime3_param= { [0, 600] }
Time 1	Time delay associated with Level 4.	s	UTime4_param= { [0, 600] }
Time 1	Time delay associated with Level 5.	s	UTime5_param= { [0, 600] }

Settings - ROCOF

The following settings are tunable:

Name	Description	Unit	Variable = {Possible Values}

Name	Description	Unit	Variable = {Possible Values}
Enable ROCOF	ROCOF element enable/disable	-	Enable_ROCOF_param= {0, 1}
Level 1	Rate-of-change-of-frequency level.	Hz/s	Rf_param= { [-10, 10] }
Time 1	Time delay associated with Level 1.	s	rocofDelay_param= { [0, 600] }

Inputs, Outputs and Signals Available for Monitoring

Inputs

Name	Description

Name	Description
Vabc	A 3-dimension voltage input for three-phase voltages, in the order of Va, Vb and Vc
Reset	The reset signal for the relay. The relay will be reset when this signal becomes 1.

Outputs

Name	Description

Name	Description
81O	The output of over-frequency relay. By default it is 0, if an over-frequency scenario is detected, it becomes 1.
81U	The output of under-frequency relay. By default it is 0, if an under-frequency scenario is detected, it becomes 1
81R	The output of rate-of-change-of-frequency relay. By default it is 0, if an under-frequency scenario is detected, it becomes 1.

Sensors

Name	Description	Unit

Name	Description	Unit
m	The calculated frequency by the relay/the actuating quantity of the relay.	Hz

Typical Values for Parameters

The parameter section shows the possible values for each parameter. These values would change to fit the situation of the 81 relay model so they cannot be taken as sole values for each parameter.

Name	Description	Unit	Typical value

Name	Description	Unit	Typical value
Frequency	The nominal frequency of the measured voltage signals. Both 50 and 60 Hz are supported in the model.	Hz	60
Freezing time	The freezing time of the 81 relay, when t < freezing time, the 81 relay is not activated.	s	0.2
Enable Overfrequency	Overfrequency element enable/disable	-	Enabled
Level 1	Over-frequency level 1.	Hz	60.5
Level 2	Over-frequency level 2.	Hz	61.0
Level 3	Over-frequency level 3.	Hz	61.5
Level 4	Over-frequency level 4.	Hz	62.0
Level 5	Over-frequency level 5.	Hz	62.5
Time 1	Time delay associated with Level 1.	s	2.0
Time 2	Time delay associated with Level 2.	s	0.5
Time 3	Time delay associated with Level 3.	s	0.4
Time 4	Time delay associated with Level 4.	s	0.3
Time 5	Time delay associated with Level 5.	s	0.2
Enable Underfrequency	Underfrequency element enable/disable	-	Enabled
Level 1	Under-frequency level 1.	Hz	59.5
Level 1	Under-frequency level 2.	Hz	59.0
Level 1	Under-frequency level 3.	Hz	58.5
Level 1	Under-frequency level 4.	Hz	58.0
Level 1	Under-frequency level 5.	Hz	57.5
Time 1	Time delay associated with Level 1.	s	2.0
Time 1	Time delay associated with Level 2.	s	0.5
Time 1	Time delay associated with Level 3.	s	0.4
Time 1	Time delay associated with Level 4.	s	0.3
Time 1	Time delay associated with Level 5.	s	0.2
Enable ROCOF	ROCOF element enable/disable	-	Enabled
Level 1	Rate-of-change-of-frequency level.	Hz/s	0.3

81 -Frequency Relay